Two general methods have been used to study the nature of Ir gene control and the nature of the antigenic determinants on protein antigens recognized by T cells and by B-cells. The first involves the comparison of responses of congenic strains of mice, differing only in their MHC haplotype, to a given antigen molecule. The second involves the comparison of the response of a single strain of mouse to a series of naturally occurring proteins or peptides differing in one or more amino acids. The use of monoclonal antibodies and T-cell clones is beginning to provide information in these area that heretofore was unattainable. In addition, recombinant DNA techniques, including site-directed mutagenesis, are being used in certain non-immunological areas to study the structure function relationships in proteins. To date little emphasis has been placed on the structural mechanisms by which the Ir gene product mediates MHC restriction. It is the purpose of the research in this proposal to combine recombinant DNA technology, especially site-directed mutagenesis, with specific monoclonal antibody and T-cell clonal probes to study the structural basis for antigen presentation and determinant selection. The mechanism by which immune responses to natural, complex antigens is regulated requires an in depth knowledge of the nature of the sites on the antigen molecule that is recognized by cells of the immune system and their physical relationship to each other on the surface of the antigen molecule. The complex molecule used in these studies is but a model for any naturally occurring antigen such as those present on infectious organisms, tumor specific antigens, etc. Studies such as those proposed here offer a new and powerful approach to the study of an old problem. The results obtained will provide considerable insight into the mechanisms of regulation of immune responses so necessary to a full understanding of disease processes and for the manipulation of immune responses in many diseases including all forms of cancer.